I have a lot of vacuum when I open the fuel cap. I know there is normaly an overpressure but in this case there is a very high vacuum, also a dent in the fueltank an dissapeares when i open the fuel cap.

A strange thing is that I discovered a lot of small pieces of plastic (grains) in the fuel filter when i changed it. I really dont know where they come from. And whats the name of the sparepart (see attachement).